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PVS 3.2 contains a number of enhancements and bug fixes.
| Installation Notes | ||
| New Features | ||
| Bug Fixes | ||
| Incompatibilities |
Installation is the same as usual. However, if you have received
patches from SRI that you have put into your ~/.pvs.lisp file,
they should be removed. If you anticipate wanting to try the newer
and older versions together, you can do this by using #-pvs3.2
in front of the patches. This is a directive to the Lisp reader, and
causes the following s-expression to be ignored unless it is an
earlier version of PVS.
The PVS startup script pvs has been made to work with later
versions of Linux (i.e., RedHat 9 and Enterprise).
There are a number of changes related to theory interpretations, as well as many bug fixes.
There is now a new form of mapping that makes it simpler to systematically interpret theories. This is the Theory View, and it allows names to be associated without having to directly list them. For example, given a theory of timed automaton:
automaton:THEORY
BEGIN
actions: TYPE+;
visible(a:actions):bool;
states: TYPE+;
enabled(a:actions, s:states): bool;
trans(a:actions, s:states):states;
equivalent(a1, s2:states):bool;
reachable(s:states):bool
start(s:states):bool;
END automaton
|
One can create a machine with definitions for actions, etc.,
and create the corresponding interpretation simply by typing
IMPORTING automaton :-> machine |
This is read as a machine viewed as an automaton, and is equivalent to
IMPORTING machine
IMPORTING automaton {{ actions := machine.actions, ... }}
|
Here the theory view was in an importing, but it is really a theory name, and hence may be used as part of any name. However, the implicit importing of the target is done only for theory declarations and importings. In all other cases, the instance needed must already be imported. Thus it is an error to reference
automaton :-> machine.start(s) |
unless machine has already been imported. This is not
very readable,(2) so it is best to introduce
a theory abbreviation:
IMPORTING automaton :-> machine AS M1a |
or a theory declaration:
M1t: THEORY = automaton :-> machine |
The difference is that M1a is just an abbreviation for an
instance of an existing theory, whereas M1t is a new copy of that
theory, that introduces new entities. Thus consider
IMPORTING automaton :-> machine AS M2a M2t: THEORY = automaton :-> machine |
The formula M1a.actions = M2a.actions is type correct, and
trivially true, whereas M1t.actions = M2t.actions is not even
type correct, as there are two separate actions declarations
involved, and each of those is distinct from machine.actions.
The grammar for Name and TheoryName has been changed to reflect the new syntax:
TheoryName := [Id ']' Id [Actuals] [Mappings] [':->' TheoryName]
Name := [Id ']' IdOp [Actuals] [Mappings]
[':->' TheoryName] ['.' IdOp]
|
The left side of :-> is called the source, and the
right side is called the target. Note that in this case the target
provides a refinement for the source.
For a given theory view, names are matched as follows. The uninterpreted types and constants of the target are collected, and matched to the types and constants of the source. Partial matching is allowed, though it is an error if nothing matches. After finding the matches, the mapping is created and typechecked.
Mapping an entity typically means that it is not accessible in the context. For example, one may have
IMPORTING T{{x := e}} AS T1
|
where the e is an expression of the current context. The x,
having been mapped, is not available, but it is easy to forget this and
one is often tempted to refer to T1.x. One possible work-around
is to use theory declarations with = in place of :=, but
then a new copy of T will be created,
which may not be desirable (or in some cases even possible - see the Theory
Interpretations Report ).
To make mappings more convenient, such references are now allowed. Thus
in a name of the form T1.x, x is first looked for in
T1 in the usual way, but if a compatible x cannot be
found, and T1 has mappings, then x is searched for in
the left sides, and treated as a macro for the right side if found. Note
that x by itself cannot be referenced in this way; the theory
name must be included.
Developing specifications and proofs often leads to the creation of definitions and lemmas that turn out not to be necessary for the proof of the properties of interest. This results in specifications that are difficult to read. Removing the unneeded declarations is not easy, as it is difficult to know whether they are actually used or not.
The new commands unusedby-proof-of-formula and
unusedby-proofs-of-formulas facilitate this. The
unusedby-proof-of-formula command creates a ’Browse’ buffer
listing all the declarations that are unused in the proof of the given
formula. Removing all these declarations and those that follow the
given formula should give a theory that typechecks and for which the
proofchain is still complete, if it was in the full theory. This
could be done automatically in the future.
PVS specifications are saved as binary (.bin) files, in order
to make restarting the system faster. Unfortunately, it often turned
out that loading them caused problems. This was handled by simply
catching any errors, and simply retypechecking. Thus in many cases
the binary files actually made things slower.
Until PVS version 3.2, binary files corresponded to the specification
files. This means that if there is a circularity in the files (i.e.,
theories A and C are in one file, B in another,
with A importing B importing C) then there is no
way to load these files. In 3.2, bin files correspond to theories.
These are kept in a pvsbin subdirectory of the current context.
However, there was a more serious problem with the binary files. It turns out that loading a binary file took more space, and the proofs took longer to run. The reason for this is that the shared structure that is created when typechecking sources is mostly lost when loading binary files. Only the structure shared within a given specification file was actually shared. In particular, types are kept in canonical form, and when shared, testing if two types are equal or compatible is much faster.
The binary files are now saved in a way that allows the shared structure to be regained. In fact, there is now more sharing than obtained by typechecking. This is one of the main reasons that this release took so long, as this forced many new invariants on the typechecker.
The payoff is that, in general, binary files load around five times faster than typechecking them, and proofs run a little faster because of the increased sharing. This is based on only a few samples, in the future we plan on systematically timing the specifications in our validation suite.
The commands html-pvs-file and html-pvs-files generate
HTML for PVS specification files. These can be generated in place, or
in a specified web location. This is driven by setting a Lisp
variable *pvs-url-mapping*, as described below.
The in place version creates a pvshtml subdirectory for each
context and writes HTML files there. This is done by copying the PVS
file, and adding link information so that comments and whitespace are
preserved. Note that there is no html-theory command. This is
not an oversight; in creating the HTML file links are created to point
to the declarations of external HTML files. Hence if there was as way
to generate HTML corresponding to both theory and PVS file, it would
be difficult to decide which a link should refer to.
HTML files can be generated in any order, and may point to library
files and the prelude. Of course, if these files do not exist then
following these links will produce a browser error. The
html-pvs-files command will attempt to create all files that
are linked to, failure is generally due to write permission problems.
Usually it is desirable to put the HTML files someplace where anybody
on the web can see them, in which case you should set the
*pvs-url-mapping* variable. It’s probably best to put this in
your ~/.pvs.lisp file in your home directory so that it is
consistently used. This should be set to a list value, as in the
following example.
(setq *pvs-url-mapping*
'("http://www.csl.sri.com/~owre/"
"/homes/owre/public_html/"
("/homes/owre/pvs-specs" "pvs-specs" "pvs-specs")
("/homes/owre/pvs3.2" "pvs-specs/pvs3.2" "pvs-specs/pvs3.2")
("/homes/owre/pvs-validation/3.2/libraries/LaRC/lib"
"pvs-specs/validation/nasa"
"pvs-specs/validation/nasa")))
|
The first element of this list forms the base URL, and is used to
create a <base> element in each file. The second element is
the actual directory associated with this URL, and is where the
html-pvs-file commands put the generated files. The rest of
the list is composed of lists of three elements: a specification
directory, a (possibly relative) URL, and a (possibly relative) HTML
directory. In the above example, the base URL is
http://www.csl.sri.com/~owre/, which the server associates with
/homes/owre/public_html. The next entry says that specs found
in (a subdirectory of) /homes/owre/pvs-specs are to have
relative URLs corresponding to pvs-specs, and relative
subdirectories similarly. Thus a specification in
/homes/owre/pvs-specs/tests/conversions/ will have a
corresponding HTML file in
/homes/owre/public_html/pvs-specs/test/conversions/ and
correspond to the URL
http://www.csl.sri.com/~owre/pvs-specs/test/conversions/.
In this case, PVS is installed in /homes/owre/pvs3.2, and thus
references to the prelude and distributed libraries (such as finite
sets), will be mapped as well. Note that in this example, all the
relative structures are the same, but it doesn’t have to be that way.
The *pvs-url-mapping* is checked to see that the directories
all exist, though currently no URLs are checked (if anybody knows a
nice way to do this from Lisp, please let us know). If a subdirectory
is missing, the system will prompt you for each subdirectory before
creating it. A n or q answer terminates processing
without creating the directory, a y creates the
directory and continues, and a ! causes it to just create any
needed directories without further questions.
If a *pvs-url-mapping* is given, it must be complete for the
file specified in the html-pvs-file command. In practice, this
means that your PVS distribution must be mapped as well. PVS will
complain if it is not complete; in which case simply add more
information to the *pvs-url-mapping* list.
No matter which version is used, the generated HTML (actually
XHTML) file contains a number of <span> elements. These simply
provide a way to add class attributes, which can then be used
in Cascading Style Sheet (CSS) files to define fonts, colors, etc.
The classes currently supported are:
span.comment span.theory span.datatype span.codatatype span.type-declaration span.formal-declaration span.library-declaration span.theory-declaration span.theory-abbreviation-declaration span.variable-declaration span.macro-declaration span.recursive-declaration span.inductive-declaration span.coinductive-declaration span.constant-declaration span.assuming-declaration span.tcc-declaration span.formula-declaration span.judgement-declaration span.conversion-declaration span.auto-rewrite-declaration |
See the <PVS>/lib/pvs-style.css file for examples. This file
is automatically copied to the base directory if it doesn’t already
exist, and it is referenced in the generated HTML files. Most browsers
underline links, which can make some operators difficult to read, so
this file also suppresses underlines. This file may be edited to suit
your own taste or conventions.
Both the html-pvs-file commands take an optional argument.
Without it, many of the common prelude operators are not linked to.
With the argument all operators get a link. Overloaded operators not
from the prelude still get links.
There is now a default-strategy that is used by the prover for
the prove-using-default commands, and may be used as a parameter in
pvs-strategies files. For example, the pvs-strategies file in
the home directory may reference this, which is set to different
values in different contexts.
While in the prover, the typechecker now checks the sequent to see if
the given expression needs to have a TCC generated. It does this by
examining the formulas of the sequent, to see if the given expression
occurs at the top level, or in a position from which an unguarded TCC
would be generated. Thus if 1/x appears in the sequent in an
equation y = 1/x, the TCC x /= 0 will not be generated.
But if the expression only appears in a guarded formula, for example,
x = 0 IMPLIES y = 1/x, then the TCC will still be generated.
This is sound, because for the expression to appear in the sequent
necessary TCCs must already have been generated. This greatly
simplifies proofs where annoying TCCs pop up over and over, and where
the judgment mechanism is too restrictive (for example, judgements
cannot currently state that x * x >= 0 for any real x).
Obviously, this could affect existing proofs, though it generally makes them much simpler.
typepred! rule and all-typepreds strategyAny given term in the sequent may have associated implicit
type constraints. When a term is first introduced to a sequent there
may be TCCs associated, either on the formula itself, or as new
branches in the proof. The term may subsequently be rewritten, but
there is still associated with the term an implicit TCC. For example,
the term 1/f(x) may be introduced, and later simplified to
1/(x * x - 1). Since f(x) was known to be nonzero, it
follows that x * x - 1 is also nonzero (in this context),
though this is not reflected in the types or judgements.
The typepred! rule has been modified to take a
:implicit-typepreds? argument, which looks for occurrences of
the given expression in the sequent, and creates the implicit type
constraint (if any) as a hypothesis. It does this only for
occurrences that are unguarded, i.e., occur positively. This is
stricter than the way TCCs are actually generated. This is needed
because, for example, conjunction is commutative, and can be rewritten
in the prover. Thus the hypothesis x /= 0 => 1/x /= x could be
rewritten to 1/x = x => x = 0, and the left-to-right reading
will generate x /= 0, which is obviously unsound. Note that
this does not mean that TCC generation or applying the rewrite is
unsound, as the TCC simply says that a type can be assigned to the
term. Technically, a TCC for a term of the form A => B could
be a disjunction (A => TCC(B)) OR (NOT B => TCC(A)), but this
is more costly in many ways, and rarely useful in practice.
Thus the command (typepred! "x * x - 1" :implicit-typepreds? t)
generates the hypothesis x * x - 1 /= 0 assuming that the term
occurs positively in a denominator.
A generally more useful strategy is all-typepreds. This
collects the implicit type constraints for each subexpression of the
specified formula numbers. This can be especially handy for
automating proofs, though there is the potential of creating a lot of
irrelevant hypotheses.
There are two new prover commands: grind-with-ext and
reduce-with-ext. These are essentially the same as
grind and reduce, but also perform extensionality. This
is especially useful when reasoning about sets.
There are new forward chain commands available:
forward-chain@, forward-chain*, and
forward-chain-theory. forward-chain@ takes a list of
forward-chaining lemmas (of the form A1 & ... & An => B, where
free variables in B occur among the free variables in the
Ai), and attempts the forward-chain rule until the first one
succeeds. forward-chain* takes a list, and repeatedly
forward-chains until there is no change; when successful it starts
back at the beginning of the list. forward-chain-theory
creates a list of the applicable lemmas of the given theory and
invokes forward-chain*.
TeX substitutions have been improved, allowing substitutions to be
made for various delimiters, as shown below. The TeX commands
are defined in the pvs.sty file at the top level of the PVS
directory. They consist of the prefix, followed by ’l’ or
’r’ to indicate the left or right delimiter.
| Name | Symbols | TeX Command Prefix | TeX |
| parentheses | ( ) | \pvsparen | |
| brackets | [ ] | \pvsbracket | |
| record type constructors | [# #] | \pvsrectype | |
| bracket bar | [| |] | \pvsbrackvbar | |
| parenthesis bar | (| |) | \pvsparenvbar | |
| brace bar | {| |} | \pvsbracevbar | |
| list constructor | (: :) | \pvslist | |
| record constructor | (# #) | \pvsrecexpr |
These can be customized either by including new mappings for the
symbols in a pvs-tex.sub file, or by overriding the TeX
commands in your LaTeX file. It may be useful to look at the default
pvs.sty and pvs-tex.sub files; both are located in the
top level of the PVS installation (provided by M-x
whereis-pvs).
The add-declaration command now allows IMPORTINGs. This is
most useful during a proof when a desired lemma is in a theory that
has not been imported. Note that it is possible for the file to no
longer typecheck due to ambiguities after this, even though the proof
will go through just fine. Such errors are typically very easy to
repair.
Although no new theories have been added, there are a number of new
declarations, mostly lemmas. These are in the theories sets,
function_inverse, relation_defs, naturalnumbers,
reals, floor_ceil, exponentiation, and
finite_sets.
The bv_cnv theory was removed, as the conversion can sometimes
hide real type errors. To enable it, just add the following line to
your specification.
CONVERSION fill[1] |
The PVS Bugs List shows the status of reported bugs. Not all of these have been
fixed as of PVS version 3.2. Those marked feedback or
closed are the ones that have been fixed. The more significant
bug fixes are described in the following subsections.
| Retypechecking | ||
| Quantifier Simplification |
PVS specifications often span many files, with complex dependencies. The typechecker is lazy, so that only those theories affected by a change will need to be retypechecked. In addition, not all changes require retypechecking. In particular, adding comments or whitespace will cause the typechecker to reparse and compare the theories to see if there was a real change. If not, then the place information is updated and nothing needs to be retypechecked. Otherwise, any theory that depends on the changed theory must be untypechecked. This means that the typechecker cannot decide if something needs to be untypechecked until it actually reparses the file that was modified.
Thus when a file is retypechecked, it essentially skips typechecking declarations until it reaches an importing, at which point it retypechecks that theory. When it reaches a theory that has actually changed, untypechecking is triggered for all theories that import the changed theory. The bug was that only the top level theory was untypechecked correctly; any others would be fully untypechecked, but since they were already in the process of being typechecked, earlier declarations would no longer be valid.
The fix is to keep a stack of the theories being typechecked and the importing they are processing, and when a change is needed, the theories are only untypechecked after the importing.
In PVS 3.1, a form of quantifier simplification was added, so that
forms such as FORALL x: x = n IMPLIES p(x) were automatically
simplified to p(n). In most cases, this is very useful, but
there are situations where the quantified form is preferable, either
to trigger forms of auto-rewriting or to allow induction to be used.
Many proof commands now include a :quant-simp? flag to control
this behavior. By default, quantifier simplification is not done;
setting the flag to t allows the simplification.
simplify, assert, bash, reduce,
smash, grind, ground, lazy-grind,
crush, and reduce-ext all have this flag.
The decision procedures have been made more complete, which means that
some proofs may finish sooner. Unfortunately, some proofs may also
loop that didn’t before (3).
This is usually due to division, and a workaround is to use the
name-replace command to replace the term involving division
with a new name, and then using the decision procedure (e.g.,
assert). If you find that the prover is taking too long, you
can interrupt it with C-c C-c, and run :bt to see the
backtrace. If it shows something like the following, then you know
you are in the ground decision procedure.
find1 <- pr-find <- chainineqs <- transclosure <- addineq <- process1 <- ineqsolve <- arithsolve <- solve <- pr-merge <- process1 <- ineqsolve <- arithsolve <- solve |
At this point, you can either run (restore) to try a different
command (like name-replace), or :cont in the hope that
it will terminate with a little more time. And yes, there are
situations where the bug is not a problem, it just takes a long time
to finish.
In the past, a name could reference the actuals of the current theory. This is actually a mistake, as the actuals were generally ignored in this case. Though this rarely caused problems, there were a few reported bugs that were directly due to this, so now the system will report that the actuals are not allowed. To fix this, simply remove the actual parameters. Note that this can affect both specifications and proofs.
In earlier versions of PVS, once a library theory was typechecked, it could be referenced without including the library id. This is no longer valid. First of all, if the given theory appears in two different libraries, it is ambiguous. Worse, if it also appears in the current context, there is no way to disambiguate. Finally, even if there is no ambiguity at all, there can still be a problem. Consider the following:
A: THEORY ... IMPORTING B, C ... END A B: THEORY ... IMPORTING lib@D ... END B C: THEORY ... IMPORTING D ... END C |
This typechecks fine in earlier versions of PVS, but if in the next
session the user decides to typecheck C first, a type error is
produced.
This has been improved, so that variables are generally named apart. In some cases, this leads to proofs failing for obvious reasons (an inst variable does not exist, or a skolem constant has a different name).
bddsimp and Enumeration TypesFixed bddsimp to return nicer formulas when enumeration types are
involved. These are translated when input to the BDD package,
but the output was untranslated. For example, if the enumeration type
is {a, b, c}, the resulting sequents could have the form
a?(x) b?(x)
|---- |---- |----
a?(x) b?(x)
a?(x)
|
With this change, instead one gets
a?(x) b?(x) c?(x) |---- |---- |---- |
Which is nicer, and matches what is returned by prop. This makes certain proofs faster, because they can use the positive information, rather than the long and irrelevant negative information. Of course, the different formula numbering can affect existing proofs.
The prettyprint-theory-instance command was introduced along
with theory interpretations, but it was restricted to theory instances
that came from theory declarations, and would simply prettyprint
these. Unfortunately, such theories are very restricted, as
they may not refer to any local declarations. The
prettyprint-theory-instance now allows any theory instance to
be given, and displays the theory with actuals and mappings
performed. This is not a real theory, just a convenient way of
looking at all the parts of the theory instance.
The visibility rules for assumings and mapped axioms has been modified. Most TCCs are generated so that the entity that generated them is not visible in a proof. This is done simply by inserting the TCCs before the generating declaration. Assuming and Mapped Axiom TCCs are a little different, in that they may legitimately refer to declarations that precede them in the imported theory. To handle this, these TCCs are treated specially when creating the context. All declarations preceding the assuming or axiom that generated the TCC are visible in the proof of the TCC.
The replace prover command now does the replacement in types as
well as expressions when the :actuals? flag is set. It is
possible, though unlikely, that this could cause current proofs to
fail. It is more likely that branches will be proved sooner.
expand Rule uses Full NameWhen the expand rule was given a full name it would ignore
everything but the id. This has been fixed, so that other information
is also used. For this command, the name is treated as a pattern, and
any unspecified part of the name is treated as matching anything.
Thus th.foo will match foo only if it is from theory
th, but will match any instance or mapping of th.
foo[int] will match any occurrence of foo of any theory,
as long as it has a single parameter matching int. The
occurrence number counts only the matching instances.
This change is only going to affect proofs in which more than just an
identifier is given to expand.
In theory finite_sets_minmax the functions min and
max defined on the type parameter have been renamed to
fsmin and fsmax, respectively. This was done because
they are only used in the definitions of min and max
over finite sets, and can cause ambiguities elsewhere.
There was a bug reported where induct was generating a large
number of subgoals; this turned out to be due to the indiscriminate
use of beta, which was intended to simplify newly added
formulas but could also affect the conclusion and subsequent
processing. To fix this, beta is now only applied to newly
generated formulas. This may make some proofs fail, though generally
they will be fixed simply by using beta after induct.
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